Combustion effluents and waste products from various installations are a major source of air pollution when discharged into the atmosphere. A particularly troublesome pollutant found in many combustion effluent streams is nitrogen oxide, designated as NO.sub.x to collectively indicate the presence of more than one oxide, a major irritant in smog. Furthermore, it is believed that the principal oxide, NO.sub.2, undergoes a series of reactions known as photo-chemical smog formation, in the presence of sunlight and hydrocarbons. The major source of NO.sub.2 is NO, which to a large degree, is generated at such stationary installations as gas and oil-fired steam boilers for electric power plants, process heaters, incinerators, coal fired utility boilers, glass furnaces, cement kilns, oil field steam generators, and gas turbines.
Heretofore, attempts at achieving NO.sub.x reduction in gas turbines via NH.sub.3 injection have failed due to the generally correct assessment that conditions very unfavorable to significant NO.sub.x reduction exist within the combustor. Namely that there are higher than optimum temperatures and very low residence times.
Thermal DeNO.sub.x technology developed at Exxon Research and Engineering teaches a method of NO.sub.x control which appears to be generally applicable to gas turbines. U.S. Pat. No. 3,900,554 to Richard K. Lyon teaches that substantial NO.sub.x reduction can be achieved in the 1600.degree. F. to 2000.degree. F. temperature range and in the presence of excess oxygen via NH.sub.3 injection. Temperatures within this range do indeed exist within the gas turbine. However, another important ingredient which is key to significant DeNO.sub.x performance in gas turbines is residence time. The above-stated patent describes a broad range of applicable residence times from 0.001 to 10 seconds, both the examples only use a 75 millisecond or above residence time carried out at 1 atmosphere pressure. Typically, residence times within the gas turbine combustor are approximately 20-30 milliseconds which is far too low to achieve required NO.sub.x reductions at atmospheric pressure.
In U.S. Air Force report AFAPL-TR-72-80 entitled "Fuel Modification for Abatement of Aircraft Turbine Engine Oxides of Nitrogen Emissions" (October, 1972) fuel additives including ammonia-water and ammonia-water-methanol were used in aircraft turbine engines operating at pressures of about 45-60 psig. The results reported showed only slight NO.sub.x reductions.
In a paper entitled "Exxon Thermal DeNO.sub.x Process for Stationary Combustion Sources" delivered May 25, 1982, the present inventor disclosed plots of ammonia and NO.sub.x concentration for utility boilers. However, the data presented does not disclose operations at superatmospheric pressure.
Adding a considerable excess of ammonia to reduce the NO.sub.x in the relatively short residence time in the turbine combustor is not desirable, since this would, to some extent, result in NO.sub.x emissions to the atmosphere being replaced by NH.sub.3 emissions. In addition, since NH.sub.3 may be oxidized to produce NO under certain conditions, injection of a large excess of NH.sub.3 into the gas turbine combustor may not result in a significant decrease in NO.sub.x emissions.
The limitation of thermal DeNO.sub.x technology as applied to gas turbines has been analyzed by C. P. Fenimore in "Combustion and Flames" Volume 37: pages 245-250 (1980). As presented by Fenimore, the NO reduction reaction by NH.sub.3 is strongly influenced by the [OH] concentration. Thus, one can increase the rate of NO reduction by increasing [OH] concentration. Unfortunately, however, increasing [OH] causes a disproportionate increase in the rate of the NH.sub.3 +OH.fwdarw.NO reaction. Thus, any effort to improve the rate of NO reduction inevitably ruins the selectivity of the reaction and one is trapped in a situation in which NH.sub.3 either reacts with good selectivity but too slowly, or it reacts fast enough but with poor selectivity, so that the net reduction of NO is poor. The effects of this conclusion were demonstrated by Fenimore based on lab tests conducted at conditions typical of gas turbine combustors but at atmospheric pressure.
In the "Proceedings of the American Flame Research Committee" 1984 International Symposium on Alternative Fuels and Hazardous Wastes, P.C.T. de Boer developed an extended NO.sub.x reaction model and used the model to assess the potential for DeNO.sub.x is a gas turbine combustor. DeBoer concluded that the residence time in the combustor was not sufficient to produce a low level of NO.sub.x in the gas turbine emissions.
In addition, H.sub.2 O concentration in the flue gas plays a major role in the DeNO.sub.x reaction via NH.sub.3 injection. This is illustrated by FIG. 1 where increasing H.sub.2 O concentration has a significant inhibiting effect on DeNO.sub.x performance. Frequently, water or steam is injected into combustors, particularly gas turbine combustors, to lower the peak exhaust gas temperature and the NO.sub.x content of the combustion effluent. Thus, one would conclude that H.sub.2 O injection into gas turbine combustors for NO.sub.x reduction is somewhat incompatible with simultaneous NH.sub.3 injection.
The analysis of model gas turbine systems to date has generally been based on laboratory DeNO.sub.x data taken at or near the specific temperature and residence time conditions within the combustor without considering elevated pressure. However, recently there has been developed a kinetic model utilizing the basic mechanism of NO.sub.x reduction via NH.sub.3 injection considering both chemistry and physical properties including pressure. This model has been validated at atmospheric pressure with an extensive bank of experiments and at elevated pressure by limited experiments. Thus, it is now possible to conduct analyses via computer calculations without the need to perform specific experiments.
Use of this kinetic model has led to the surprising discovery that at conditions normally existing at the exit of the combustor, significant DeNO.sub.x can be achieved. At high pressures common to gas turbine combustors of 10-15 atmospheres, the thermal DeNO.sub.x reaction is positively and significantly affected even at relatively low residence times. Since the kinetic model calculations represent a performance limit, calculations have also been performed via a 2-dimensional turbulent flow model combined with the kinetic model to take into account the effects of mixing and temperature profiles in the combustor. These calculations confirm that significant DeNO.sub.x can be achieved through this invention.